Stabilizer concentrate

ABSTRACT

An improved stabilizer concentrate for use in stabilizing polymeric blend compositions which comprise a thermally sensitive interpolymer. The stabilizer concentrate comprises a carrier resin, and a stabilizing agent capable of stabilizing a thermally sensitive interpolymer. The improvement comprises employing a carrier resin having a viscosity such that the stabilizer concentrate has a viscosity of less than 60 percent of the viscosity of the polymeric blend composition.

This application is a continuation in part of application Ser. No. 019,169 filed Feb. 26, 1987, now abandoned.

The present invention concerns an improved stabilizer concentrate for use in polymeric blend compositions. Specifically, the present invention concerns an improved stabilizer concentrate for use in polymeric blend compositions which blend compositions comprise a thermally sensitive polymeric material, particularly, a thermally sensitive interpolymer, for example a vinylidene chloride interpolymer and at least one other polymeric component.

Vinylidene chloride interpolymers are well known in the prior art for their excellent barrier to the mass transport of atmospheric gases and moisture vapor. Because of their excellent barrier properties, vinylidene chloride interpolymers are well suited for use in food packaging operations and other similar operations, wherein it is desirable to protect a given article from oxygen and loss of moisture.

In forming containers suitable for use in food packaging applications, it is common to employ a layer of thermally sensitive barrier polymer in a multi-layer structure, thus providing a structure which is possessed of good oxygen and water vapor barrier properties. For example, it has proven desirable to form barrier containers and films out of a multi-layer structure comprising two outer layers of a polymeric material possessed of desirable processing properties and at least one inner layer of a vinylidene chloride interpolymer. Such multi-layer structures may be formed through laminating processes or through a process of coextrusion or other similar process which are well known in the art.

Processes for forming and using multi-layer structures such as those described above inherently produce a certain amount of scrap material. From an economic standpoint, it is desirable to employ this scrap material in a useful manner. In the past, this has proven difficult since the scrap material comprises a thermally sensitive polymer and at least one other polymer which usually forms the outer layers of the multi-layer structures. Typically, the polymeric material which comprises the non-barrier layers of the multi-layer structure has processing properties which are quite different from the thermally sensitive barrier polymer. Attempts to reprocess or recycle the scrap material have proven to be difficult and somewhat unsatisfactory because reprocessing of the scrap material typically leads to an unacceptable degree of degradation of the thermally sensitive barrier polymer.

The present invention concerns an improved stabilizer concentrate for use in stabilizing a polymeric blend composition which contains a thermally sensitive polymeric material, particularly an interpolymer, and at least one other polymeric component, the blend containing less than 90 volume percent of the thermally sensitive polymer material, the stabilizer concentrate comprising a blend of:

(1) a carrier resin, and

(2) a stabilizing agent capable of stabilizing the thermally sensitive polymeric material; the concentrate having a viscosity of less than 60 percent of the viscosity of the recycle polymeric blend composition.

Additionally, the present invention concerns an improved process or preparing recyclable polymeric blends containing a thermally sensitive polymeric material, particularly an interpolymer, and at least one other polymeric component. The improved process comprising the steps of intimately admixing the polymeric blend composition with a stabilizer concentrate which comprises a blend of:

(1) a carrier resin, and

(2) a stabilizing agent capable of

stabilizing a thermally sensitive polymeric material, particularly an interpolymer; wherein the improvement comprises employing a carrier resin having a viscosity such that the stabilizer concentrate has a viscosity of less than 60 percent of the viscosity of the polymeric blend composition.

For the purpose of this invention, it is understood that the term "thermally sensitive polymeric material" refers to any homopolymer or interpolymer which exhibits an undesirable change in properties, particularly physical properties, upon exposure to desirable processing temperatures, In many instances the thermally sensitive interpolymer will be a barrier polymer. Exemplary of suitable barrier polymers are vinylidene chloride interpolymers, ethylene/vinyl alcohol interpolymers, polychlorotrifluoroethylene, poly(vinylidene fluoride), poly(ethylene terephthalate), acrylonitrile (>50 weight percent)/styrene copolymers, poly(phenylene oxide), poly(vinyl chloride), and the like. The preferred thermally sensitive barrier polymers are vinylidene chloride interpolymers.

For the purposes of this invention, it is understood that the term "vinylidene chloride interpolymer" encompasses both homopolymers and interpolymers of vinylidene chloride. The vinylidene chloride interpolymers are suitably formed from a monomer mixture comprising vinylidene chloride in an amount of from 40 to 100 percent, beneficially from 50 to 96 percent, and desirably from 60 to 94 percent by weight of total monomer mixture. When the monomer mixture does not comprise 100 weight percent vinylidene chloride, the monomer mixture comprises one or more monoethylenically unsaturated monomers which are copolymerizable with the vinylidene chloride monomer. The amount of monoethylenically unsaturated monomer is suitably from 0 to 60 weight percent, beneficially from 4 to 50 weight percent, and desirably from 6 to 40 weight percent based on total weight of the monomer mixture.

Monoethylenically unsaturated monomers suitable for copolymerization with vinylidene chloride include vinyl chloride, alkyl acrylates, alkyl methacrylates, acrylic acid, methacrylic acid, itaconic acid, acrylonitrile, and methacrylonitrile. The monoethylenically unsaturated monomers are desirably selected from the group consisting of vinyl chloride, alkyl acrylates, and alkyl methacrylates, the alkyl acrylates and alkyl methacrylates having from 1 to 8 carbon atoms per alkyl group. The alkyl acrylates and alkyl methacrylates preferably have from 1 to 4 carbon atoms per alkyl group. The alkyl acrylates and alkyl methacrylates are most preferably selected from the group consisting of methylacrylate, ethylacrylate, and methyl methacrylate.

In one preferred embodiment of the present invention, the vinylidene chloride interpolymer is formed from a monomer mixture which comprises vinylidene chloride in an amount of from 75 to 85 weight percent and vinyl chloride in an amount of from 15 to 25 weight percent based on total weight of the monomer mixture.

In a second preferred embodiment of the present invention, the vinylidene chloride interpolymer is formed from a monomer mixture which comprises vinylidene chloride in an amount of from from 92 to 96 weight percent and methylacrylate in an amount of from 4 to 8 weight percent, based on total monomer mixture weight.

The polymeric blend compositions suitably stabilized by the stabilizer concentrates of the present invention comprise at least one polymeric resin other than the thermally sensitive barrier polymers described above. A wide variety of other polymeric resins may be employed in forming the polymeric blend composition. As described above, the polymeric resin other than the thermally sensitive barrier polymer is generally present and selected for its physical properties and processing characteristics. As a general rule, any polymer beneficially used to form multi-layer barrier containers in conjunction with a thermally sensitive barrier polymer is suitably used in the polymeric blend compositions.

The polymeric blend compositions advantageously comprise from 0.5 o 90 volume percent of the thermally sensitive interpolymer, preferably from 2 to 70 volume percent and most preferably from 10 to 40 volume percent.

Polymeric resins suitable for the use in the polymeric blend compositions stabilized by the stabilizer concentrates of the present invention include, for example, the following polymeric resins: polypropylene, polystyrene, polyethylene, polybutylene, interpolymers of two or more α-olefins having from 2 to 18 carbon atoms per molecule, styrene (>50 weight percent)/acrylonitrile copolymers, impact modified polystyrene, polyethyleneterephthalate, polycarbonates, and ethylene-vinylacetate copolymers.

The stabilizer concentrates of the present invention comprise a blend of:

(1) a carrier resin, and

(2) a stabilizing agent capable of stabilizing a thermally sensitive barrier polymer, wherein the carrier resin has a viscosity such that the stabilizer concentrate has a viscosity of less than 60 percent of the viscosity of the polymeric blend composition.

The viscosity of the polymeric blend composition will generally be between about 1×10² and 1×10⁵ poise measured at 210° C. and as shear rate of 100 sec⁻¹. Thus the concentrate, having a viscosity of less than 60 percent of the viscosity of the polymeric blend composition, will have a viscosity of between about 0.6×10² and 6×10⁴ poise measured at 210° C. and as shear rate of 100 sec⁻¹.

The carrier resins suitable for use in forming the stabilizing concentrates of the present invention are those resins other than thermally sensitive barrier polymers previously described, which are suitable for use in forming the polymeric blend compositions. Exemplary of such resins are polypropylene, polyethyleneterephthalate, polycarbonates, polystyrene, polyethylene, polybutylene, copolymers of two or more α-olefins, having from 2 to 18 carbon atoms per molecule, styrene/acrylonitrile copolymers, impact modified polystyrenes, polycarbonates, and ethylenevinylacetate copolymers.

In the present invention, the viscosity of the polymeric blend composition is inversely related to the melt flow rate of the polymer comprising the major portion thereof. The viscosity of the stabilizer concentrate is inversely related to the melt flow rate of the carrier resin. Therefore, by changing the melt flow rate of the carrier resin the viscosity of the stabilizer concentrate changes relative to the viscosity of the polymeric blend composition.

As a general rule, polymers having a relatively high melt flow rate will have a relatively low molecular weight. Polymers having a relatively high melt flow rate, may either be prepared directly through a process which products a relatively low molecular weight polymer or may be formed by molecular weight degradation of a polymer having a relatively higher molecular weight.

The melt flow rates of polymers are conveniently determined by American Society of Testing and Materials Test Method D-1238. The viscosities of the compositions may be determined by American Society of Testing and Materials Test Method D-1703.

The stabilizing agent suitable for use in the present invention must be capable of stabilizing a thermally sensitive barrier polymer so that the polymer will exhibit a decrease in undesirable physical property changes upon exposure to desirable processing temperatures. Such stabilizing agents are well known in the prior art. Exemplary of said stabilizing agents are tetrasodiumpyrophosphate, magnesium oxide, calcium carbonate and other inorganic salts. When the thermally sensitive barrier polymer is a vinylidene chloride interpolymer, the stabilizing agent is, preferably, tetrasodiumpyrophosphate.

Methods of forming the blend of a carrier resin and stabilizing agent are well known in the prior art. Typically, the carrier resin and the stabilizing agent are blended in the melt using conventional melt processing techniques. Conventional melt processing equipment which may be used includes heated two roll compounding mills, Brabender mixers, Banbury mixers, single screw extruders, twin screw extruders, and the like. It is desirable that the carrier resin and the stabilizing agent be blended under conditions and for a time sufficient to produce a visually homogenous blend of carrier resin and stabilizing agent.

The stabilizer concentrate of the present invention may comprise other additives and fillers well known to those skilled in the art. Similarly, a variety of additives may be present in the polymeric blend compositions suitably stabilized by the stabilizer concentrates of the present invention. Exemplary of such additives are plasticizers, heat stabilizers, light stabilizers, pigments, processing aids and lubricants.

The stabilizer concentrates of the present invention suitably comprise carrier resin in an amount of from 25 to 75 weight percent and stabilizing agent in an amount of from 25 to 75 weight percent, based on total weight of the stabilizer concentrates. Beneficially, the stabilizer concentrates of the present invention comprise carrier resin in an amount of from 30 to 40 weight percent and stabilizing agent in an amount of from 60 to 70 weight percent based on total weight of the stabilizer concentrates.

The stabilizer concentrates of the present invention and the polymeric blend compositions are melt blended according to conventional melt processing techniques employing typical melt processing equipment. Exemplary of the typical melt processing equipment suitable for use in the present invention are heated two roll compounding mills, Brabender mixers, Banbury mixers, single screw extruders, twin screw extruders, and the like.

The stabilizer concentrates of the present invention are typically blended with the polymeric blend compositions in an amount sufficient to provide from 1 to 10, beneficially, from 1 to 6, and preferably from 2 to 5 weight percent of the stabilizing agent in the blend of stabilizer concentrate and polymeric blend composition. The amount of stabilizing agent present in polymeric blend compositions stabilized according to the present invention is dependent on the composition of the polymeric blend composition and the processing conditions to which said polymeric blend compositions are exposed.

It is desirable that the carrier resin be compatible with the polymer other than the thermally sensitive barrier polymer present in the polymeric blend composition. Those skilled in the art recognize that this allows for better mixing between the stabilizer concentrate and the polymeric blend composition. This is easily accomplished by having the carrier resin be present in the polymeric blend composition. For example, if the polymeric blend composition comprises a vinylidene chloride interpolymer and polypropylene, it is desirable that the carrier resin be polypropylene.

The present invention is illustrated in further detail by the following examples and comparative examples. All parts and percentages are by weight unless otherwise specifically noted.

The following procedure is used to generate the data for all Examples and Comparative Examples.

Stabilizer Concentrate Preparation

A stabilizer concentrate is prepared by melt blending 67 weight percent of tetrasodiumpyrophosphate with 33 weight percent of a polypropylene resin. The melt blending occurs in a Leistritz twin screw extruder. The stabilizer concentrate is extruded into a strand, cooled in a water bath, pelletized, and dried.

Four stabilizer concentrates are prepared. The concentrates are identical except that in each of the four concentrates a different polypropylene is used. The polypropylenes differ in viscosity. Table I sets forth the stabilizer concentrate number, the melt flow rate of the polypropylene employed therein, and the viscosity of the stabilizer concentrate.

                  TABLE I                                                          ______________________________________                                         Stabilizer                                                                     Concentrate   MFR.sup.1 of                                                     Number        Polypropylene                                                                              Viscosity.sup.2                                      ______________________________________                                         1*             35         8.5 × 10.sup.3                                 2             100         2.8 × 10.sup.3                                 3             325         3.0 × 10.sup.3                                 4             450         4.1 × 10.sup.3                                 ______________________________________                                          *Not an example of the present invention.                                      .sup.1 Melt flow rate as determined by American Society of Testing and         Materials Test Method D1238, in grams per 10 minutes.                          .sup.2 Viscosity, in poise, at 210° C. according to American            Society of Testing and Materials test Method D1703, at a shear rate of 10      seconds.sup.-1.                                                          

Data Generation

A five layer, coextruded structure is prepared. The five layer structure has a central barrier layer having two generally planar, generally parallel outer surfaces. Disposed on each of the generally planar, generally parallel outer surfaces of the central barrier layer is an adhesive layer, each adhesive layer having disposed thereon an outer skin layer.

The specific composition of the central barrier layer varies and is set forth in Table II.

                  TABLE II                                                         ______________________________________                                         Barrier Layer                                                                  Number   Composition                                                           ______________________________________                                         1        A vinylidene chloride/vinyl chloride copolymer                                 formed from a monomer mixture comprising 80                                    weight percent vinylidene chloride and 20 weight                               percent vinyl chloride, based on total monomer                                 mixture weight.                                                       2        A vinylidene chloride/methyl acrylate copolymer                                formed from a monomer mixture comprising 94                                    weight percent vinylidene chloride and 6 weight                                percent methylacrylate, based on total monomer                                 mixture weight.                                                       ______________________________________                                    

The adhesive layer comprises a blend of an ethylene/vinyl acetate copolymer with polypropylene. The adhesive composition comprises 70 weight percent of the ethylene/vinyl acetate copolymer and 30 weight percent of polypropylene, based on total adhesive composition weight. The adhesive is available from A. Schulman, Inc. The outer skin layer is formed from polypropylene commercially available from Exxon, Inc. under the trade designation Exxon 4092. The polypropylene has a melt flow rate of 2.5.

The five layer structure comprises 86 volume percent polypropylene, split equally between the two outer skin layers, four volume percent of the adhesive split equally between the two adhesive layers, and ten volume percent of the barrier layer material.

Data generated from the five layer structure described above constitutes Pass No. 0, and acts as a reference point as it contains no stabilizer concentrate.

The five layer structure prepared as described above is then ground in a granulator to produce recycle material in the form of flakes. The recycle material thus prepared is then incorporated into a seven layer coextruded structure. The seven layer coextruded structure comprises a central barrier layer having two generally parallel, generally planar outer surfaces, said outer surfaces having disposed thereon an adhesive layer, the outer surfaces of each adhesive layer having disposed thereon a recycle layer, the outer surface of each recycle layer having disposed thereon an outer skin layer. The composition of the central barrier layer, the adhesive layers and the outer skin layers are the same as set forth above in connection with the five layer structure. Except for the addition of the stabilizer concentrate, the recycle layer employed has the same composition of the entire five layer structure previously described. That is, the five layer structure of Pass No. 0 is ground into flakes and serves as the recycle layer of the seven layer structure just described. This seven layer structure constitutes Pass No. 1.

The recycle stream has blended therewith, prior to being extruded into the seven layer structure, 4.5 weight percent of a stabilizer concentrate according to the present invention. The seven layer structure comprises ten volume percent of the central barrier layer, four volume percent of the adhesive layer, split equally between the two adhesive layers, 50 volume percent of the recycle material, split equally between the two recycle layers, and 36 volume percent of the polypropylene, split equally between the two outer skin layers.

In a similar manner, six more seven layer coextruded structures are prepared as described above (Pass Nos. 2-7) with the only difference being that the recycle layer in each of the seven layer structures has the same composition as the seven layer structure designated by the preceding Pass Number with the exception of the addition of stabilizer concentrate. That is, the recycle layer of Pass No. 5 has the same composition as the entire seven layer structure of Pass No. 3; and the recycle layer of Pass No. 5 has the same composition as the entire seven layer structure of Pass No. 4, except each of the recycle layers has blended therewith, prior to being extruded into the seven layer structure, 4.5. weight percent of a stabilizer concentrate according to the present invention.

The viscosity of the recycle layer in each pass remains approximately the same and is about 7.5×10³ poise at a temperature of 210° C. and a shear rate of 100 sec⁻¹. Other test methods may be employed, it being only necessary that the viscosity ratios meet the requirement that the viscosity of the concentrate be less than 60 percent the viscosity of the blend.

Determination of Color Change

Each of the seven layer structures designated by a separate Pass No. is compared with the initial five layer structure to determine the color change between the initial five layer structure and the seven layer structure. The degree of color change is an indication of the degradation occurring in the recycle layers caused by exposure to excessive heat during processing of the polymer. The color change is determined by the Hunter Color Index as set forth in the American Society of Testing and Materials Test Method D-2244-79 as described in more detail by Richard S. Hunter, in "Photoelectric Color Difference Meteer" Volume 48 of the Journal of the Optical Society of America, pages 985-995, December, 1958. As described in the cited references, the total color change is designated by ΔE_(a),b.

Examples and Comparative Examples

The total color change and specific compositions tested are set forth in Table III.

                                      TABLE III                                    __________________________________________________________________________     Run Stabilizer                                                                            Barrier                                                             Num-                                                                               Concentrate                                                                           Layer                                                                               ΔE.sub.a,b of Pass Number:                               ber Number.sup.1                                                                          Number.sup.2                                                                        0* 1  2  3  4  5  6  7                                         __________________________________________________________________________     1*  1      1    N/A                                                                               23.9                                                                              35.4                                                                              40.9                                                                              44.8                                                                              48.4                                                                              47.6                                                                              46.9                                      2   3      1    N/A                                                                               15.9                                                                              30.5                                                                              38.9                                                                              39.9                                                                              42.3                                                                              45.1                                                                              43.2                                      3   3      2    N/A                                                                               25.2                                                                              34.6                                                                              41.4                                                                              45.4                                                                              46.9                                                                              49.2                                                                              50.3                                      4*  1      2    N/A                                                                               27.0                                                                              35.4                                                                              45.6                                                                              52.0                                                                              53.7                                                                              54.9                                                                              55.3                                      __________________________________________________________________________      *Not an example of the present invention.                                      .sup.1 From Table I                                                            .sup.2 From Table II                                                           N/A Not applicable, Pass No. 0 comprises a five layer structure containin      no recycle, and functions as a reference.                                

In order to determine the effect of the viscosity of the carrier resin on stabilizer concentrate efficiency, ΔE_(a),b data is generated for Pass No. 7 of a seven layer structure as described above, wherein carrier resins having different melt flow rates are employed. The exact compositions used and results are set forth in Table IV.

                  TABLE IV                                                         ______________________________________                                                 Stabilizer Barrier   ΔE.sub.a,b                                  Run     Concentrate                                                                               Layer     Pass  Viscosity                                   No.     No..sup.1  No..sup.2 No. 7 Ratio.sup.3                                 ______________________________________                                          4*     1          1         28.9  1.13                                        5       2          1         20.6  0.37                                        6       3          1         20.3  0.4                                         7       4          1         19.6  0.55                                        ______________________________________                                          *Not an example of the present invention.                                      .sup.1 From Table I                                                            .sup.2 From Table II                                                           .sup.3 Stabilizer concentrate viscosity (Table I) divided by recycle laye      viscosity (7.5 × 10.sup.3 poise, at a shear rate of 100 seconds          .sup.-1).                                                                

As can be seen from the above results, employing a carrier resin having a melt flow rate such that the stabilizer concentrate has a viscosity of less than 60 percent of the viscosity of the polymeric blend compositions (viscosity ratio less than 0.6) produces a stabilizer concentrate having greatly improved efficiency. 

What is claimed is:
 1. An improved process for stabilizing a polymeric blend composition containing a vinylidene chloride interpolymer and at least one other polymer, which process comprises forming an intimate admixture of the polymeric blend composition and a stabilizing concentrate which stabilizing concentrate contains a blend of: (1) a carrier resin, and (2) a stabilizing agent capable of stabilizing the vinylidene chloride interpolymer, said stabilizer concentrate having a viscosity of less than 60 percent of the viscosity of the polymeric blend composition.
 2. The improved process of claim 1, wherein the carrier resin is selected from the group consisting of polypropylene, polyethylene, polystyrene, and impact modified polystyrene.
 3. The improved process of claim 1, wherein the stabilizing agent is selected from the group consisting of tetrasodiumpyrophosphate, magnesium oxide, and calcium carbonate.
 4. The improved process of claim 1, wherein the stabilizer concentrate comprises a carrier resin in an amount of from about 25 to about 75 weight percent and a stabilizing agent in an amount of from about 25 to about 75 weight percent based on total weight of the stabilizer concentrate.
 5. The iimproved process of claim 4, wherein the stabilizer concentrate comprises a carrier resin in an amount of from about 30 to about 40 weight percent and a stabilizing agent in an amount of from about 60 to about 70 weight percent based on total weight of the stabilizer concentrate.
 6. The imroved process of claim 1, wherein the polymeric blend composition containing vinylidene chloride interpolymer is recycle material from a multi-layer structure. 